Arsenic (As) is a type of highly toxic substance, which is widely distributed in environment due to both natural and anthropogenic sources. For health concerns, the allowable arsenic level must be strictly regulated. In this study, an ultrasensitive electrochemical biosensor for arsenic contamination is developed with signal amplification mediated by hybridization chain reaction (HCR) and RecJ(f) exonuclease catalyzed reaction. DNA assembly is firstly achieved on the surface of gold electrode prior to the analysis, which generates tremendous charge-transfer resistance (R-ct). In the presence of As3+, aptamer sequence specifically binds As3+ and DNA dissociation occurs. The release of HCR product significantly decreases R-ct, which could be further enhanced by RecJf exonuclease catalyzed digestion. Superior analytical performance for As3+ detection is obtained including the limit of detection (LOD) as low as 0.02 ppb and a wide linear range from 0.1 to 200 ppb, which complies with WHO regulation. It can be envisioned that in combination with a portable electrochemical instrumentation, the sensing strategy is suitable for field application of arsenic contamination monitoring.